This invention relates generally to a circuit for protecting a second electronic circuit from reverse bias voltage conditions, and more specifically to a circuit for inhibiting the flow of destructive currents through the second electronic circuit under reverse bias voltage conditions.
All integrated circuits (ICs) require a power supply having a potential difference for use in powering internal integrated circuit components to ensure their operation. In some cases it is possible to inadvertently reverse the bias of the applied potential difference. For example in the automotive industry a battery may be connected backwards to a circuit, with the negative supply coupled to the positive power rail, and the positive supply coupled to the negative power rail; without having any form of reverse bias protection between the integrated circuit and the applied potential difference damage could result to the integrated circuits coupled thereto.
Big pass transistors are widely used in IC design to allow for large currents to flow within the integrated circuit. If a reverse bias condition occurs when a power supply is connected backwards to the unprotected IC, damage may result to the IC.
Current methods of reverse bias protection employee current-limiting resistors, diodes or MOS-transistors in series with the big pass transistors. However, the series current-limiting resistors, diodes, or MOS transistors have to pass the same amount of large current as the pass transistor. These components may cause undesired voltage drops.
For instance a known reverse bias protection technique is to place a high current discrete diode in series between the power source and the positive power supply terminal that goes to the ICs requiring protection. As a result reverse voltage from the battery simply reverse biases the diode and protects the ICs. However, the voltage drop on the diode reduces the actual DC voltage available to the IC.
It is also known to use a MOSFET driver between the positive terminal of a device and a positive supply terminal as a high side voltage switch for reverse bias protection. In this arrangement when the MOSFET is conducting a positive voltage is coupled to the positive terminal of the IC., and when the MOSFET is not conducting in the reverse biased condition, it provides reverse battery protection to the IC by shorting the positive supply voltage to ground.
For instanced a Prior Art circuit featuring reverse bias protection is shown in U.S. Pat. No. 5,539,610, Williams et al., and Prior Art FIG. 1. Here a power MOSFET is connected in series with a battery driven load. The MOSFET""s gate is driven by a xe2x80x9cfloatingxe2x80x9d driver that is connected across the terminals of the battery via a high resistance signal path incapable of high reverse currents. The gate driver contains a device that shorts the gate to the source of the MOSFET, thereby turning it off, if the battery is reversed.
In U.S. Pat. No. 5,517,379, Williams et al., Prior Art FIGS. 2a, 2b, an alternative reverse bias protection technique is shown using a MOSFET coupled to a power source with the drain connected to a load. The gate of the MOSFET is driven by a charge pump control IC in combination with a depletion mode MOSFET. The depletion mode MOSFET is connected across the source and gate terminals of the power MOSFET. When the battery is properly connected, the charge pump biases the gate of the power MOSFET so as to turn it ON, and the depletion mode MOSFET is turned OFF. When the battery is reversed, the depletion mode MOSFET is turned ON which shorts the gate and source of the power MOSFET, thereby turning the power MOSFET OFF.
In yet another U.S. Pat. No. 5,546,264, Williamson, et alxe2x80x94Prior Art FIG. 4xe2x80x94yet another reverse bias protection technique is presented using a MOSFET transistor. The MOSFET source S contact is connected to the positive battery terminal (B+). The drain D contact is connected to the positive input terminal. One end of a resistor 212 is connected to the gate G contact. The other end is connected to a high voltage terminal (B++), where the high voltage is produced by the electronics 204 in a typical manner.
Unfortunately many of these prior art reverse bias protection circuits requires some form of High Side Voltage controller for driving the gate of the MOSFET, or a charge pump circuit to actively bias the MOSFET into conduction by ensuring the gate voltage exceeds the source voltage, and finally a Floating Gate Driver, as well as a Low Side Controller. As a result a number of electrical components are required for this additional circuitry.
There is a need to provide a reverse bias protection circuit for protecting analog and digital integrated circuits, without current-limiting resistors, diodes, MOS-transistors, charge pumps, or high side drivers in addition to the transistors; consequently reducing the number of voltage drops by minimizing the number of components while still ensuring reverse bias protection for the integrated circuit coupled thereto.
In accordance with the invention there is provided a polarity sensitive electrical circuit for protecting an electrical device coupled thereto from a reverse bias input voltage condition, the electrical circuit comprising:
a positive supply terminal;
a ground terminal;
a first big pass transistor having a bulk, a gate, a source coupled to the positive supply terminal, and a drain resistively coupled to the ground terminal; and
a second protection transistor including:
a bulk coupled with the bulk of the first big pass transistor to form a coupled bulk, a source coupled with the positive supply terminal and with the source of the first big pass transistor to form a coupled source,
a gate electrically coupled to the ground terminal, and
a drain coupled to the coupled bulk and forming a first parasitic diode in a forward bias from the drain of the first big pass transistor to the coupled bulk and a second parasitic diode from the coupled bulk to the coupled source;
wherein when said first big pass transistor is in a conductive state when a voltage is provided across the positive supply terminal and the ground terminal in a first predetermined polarity and wherein said first big pass transistor is in a non-conductive state when a voltage is provided across the positive supply terminal and the ground terminal in a second other predetermined polarity.
In accordance with another aspect of the invention there is provided a polarity sensitive electrical circuit for protecting an electrical device coupled thereto from a reverse bias input voltage condition, the electrical circuit comprising:
a positive supply terminal;
a ground terminal;
a first big pass transistor having a bulk, a gate, a source coupled with the positive supply terminal and a drain resistively coupled with the ground terminal and forming a first parasitic diode in a forward bias from the drain to the bulk; and
a protection diode having an anode coupled to the source and a cathode coupled to the bulk of the first big pass transistor;
wherein said big pass transistor is in a conductive state when a voltage is provided across the positive supply terminal and the ground terminal in a first predetermined polarity and wherein current follow through the first big pass transistor is inhibited due to reverse biased current flow through the protection diode.
In accordance with yet another aspect of the invention there is provided a polarity sensitive electrical circuit for protecting an electrical device coupled thereto from a reverse bias input voltage condition, the electrical circuit comprising:
a positive supply terminal;
a ground terminal;
a first big pass transistor having a bulk, a gate, a source coupled to the positive supply terminal and a drain; and
a second protection transistor including:
a bulk coupled with the bulk of the first big pass transistor and to the drain of the first big pass transistor to form a coupled bulk,
a source coupled with the coupled bulk,
a gate resistively coupled to the positive supply terminal, and
a drain coupled to the ground terminal;
wherein when said first big pass transistor is in a conductive state when a voltage is provided across the positive supply terminal and the ground terminal in a first predetermined polarity and wherein said first big pass transistor is in a non-conductive state when a voltage is provided across the positive supply terminal and the ground terminal in a second other predetermined polarity.